The invention relates to stock material and miscellaneous articles, but more particularly, the invention relates to fiber preparation for a generally dry fibrated admix that is dispersible into a polymeric or preferably, a viscoelastic mass of the heat curable rubber type.
Discontinuous fibers have long been used as fillers or reinforcements for polymer and elastomeric materials. The fibers may be non-regenerated cellulose such as cotton or fiberized wood, or fiberized synthetic filaments such as nylon, aramid, polyester or glass. Cellulosic or synthetic fibers have not only been used as fillers, but they have also been used as reinforcements in rubber articles such as tires, hose and power transmission belts. As particularly discussed in U.S. Pat. Nos. 3,697,364 and 3,709,845, it is very difficult to mix fibers into massed rubber with good dispersion because the fibers tend to cling together in bundles or prills rather than being homogeneously dispersed into the rubber.
The above cited references teach improvements for dispersing and bonding discontinuous cellulosic fibers, or mixtures thereof, in a viscoelastomeric matrix to yield a vulcanized composite with increased Young's modulus. As taught thereby, fiber dispersity in rubber is enhanced by a process that involves slurrying fibers in liquid latex and then coagulating the latex on the fibers. The coagulation is filtered and dried leaving "pellets" or "curd" agglomerations of latex encapsulating and bonding together clusters of unoriented fibers. The object of the latex coating on the fiber clusters is to reduce fiber-to-fiber interactions and fiber breakage when the pellets are dispersed in a viscoelastomer with an intensive mixing means such as a cold-feed extruder, Banbury mixer or mill.
While it may be easy to disperse the fibrated pellets into a viscoelastomer, the cluster of fibers within each pellet are not evenly or easily dispersible either within the pellet or from the pellet to the elastomeric matrix. Coagulation drying partially cures the latex forming hard fiberized pellets which are not easily broken down during viscoelastomer mixing. The hard pellets are easier to break down in high viscosity viscoelastomeric compounds such as disclosed in the above-mentioned patents. This is because the higher viscosity stocks impart more shear to the pellets during mixing than lower viscosity elastomers such as normally used in hose, tires or power transmission belts.
The reduced dispersibility of the encapsulated fiber clusters of the pellets hampers fiber orientation during mixing into a viscoelastic elastomer and affects the physical properties of the cured fibrated elastomer (such as tensile strength and Young's modulus). The physical properties become dependent on generally unbroken fibers and an empirical relationship of fiber aspect ratio. The pellets inhibit the rheological properties by impeding flow of the mixed rubber stock such as during molding or extruding. The disuniformity of fiber dispersion in a cured elastomer is characterized by a pocked appearance in the surface of a finished product when it is subjected to strain.
The characterization of a fiberized rubber stock by measurements such as a high Young's modulus, an increase of matrix modulus and high tensile to break, are not always critical parameters relied on by those skilled in the art of designing and manufacturing such rubber articles as hose, tires, or power transmission belts. This is primarily because such rubber articles are made using elastomeric materials that do not follow Hooke's law where stress is proportional to strain and on which Young's modulus is based. Also, such rubber articles are designed to operate at stress levels substantially below maximum tensile to break or at large percentage elongation. Physical properties of fiber/elastomeric composites may better be characterized by more traditional elastomer technology terms such as secant modulus (e.g. stress at a specified elongation, preferably 5 to 10% for fiber composites); or comparative shapes of stress-strain curves for different fiber composite elastomers, in combination with a ratio of projected area of the stress-strain curves to indicate work capacity and fatigue life. However, a comparative evaluation of "tensile to yield" is a good expedient for evaluating elastomer-fiber adhesive systems.
Accordingly, the attainment of a process which yields an improved use of fiber in elastomers is an important advance in the art.